Aircraft commonly have controls that enable the pilot to “trim” the pitch, i.e., the “nose up”/“nose down” attitude of an aircraft. A “nose up” pitch generally means the aircraft is increasing in altitude, whereas a “nose down” pitch generally means the aircraft is decreasing in altitude. Keeping the aircraft's pitch in a neutral attitude when the controls are in a neutral position is typically desirable.
One way to do this is to “trim” the pitch based on the current airspeed, thrust and other factors so the neutral position of the yoke or other inceptor provides level flight with no moment forcing the nose up or down. Trimming neutralizes the force required to keep control surfaces in a specific position. A properly trimmed aircraft relieves the pressure on the elevator control, or the force the pilot must exert to keep the plane level. Using the natural stability of the airplane in combination with the pitch trim can permit much smoother flight control and reduced pilot workload.
Most aircraft have a pitch trim control that sets a “nose up”, “nose down” or neutral pitch when the yoke (which typically controls the elevator and/or other pitch control surface) is in its neutral position. In small aircraft, pitch trimming is usually accomplished by means of moveable trim tabs on the elevator control surfaces which are controlled by a trim wheel or other trim control in the cockpit. For example, rotating the tab downward develops an upward lift on the trim tab, which rotates the elevator up to a desired trim angle. With proper adjustment of the trim tab, the elevator moves to the required trim angle for the desired speed, and the force on the control stick required to maintain desired pitch attitude goes to zero. In larger aircraft, pitch trimming is often performed by a processor operating a motor to control the angle/position of the horizontal stabilizer, to which the elevator is attached (see FIG. 1).
While pitch trim can be adjusted after the aircraft is underway, it is often set at takeoff to establish a nose up/nose down bias to account for where the center of gravity (CG) of the aircraft is located. In this context, “CG” means the average location of the center of gravity of the aircraft (i.e., the center of mass of the overall airplane)—which has a particular known location for an empty airplane but can vary with amount of fuel, passenger/cargo loading and other factors.